Weighing in at 68kg , Prashant Kumar, left, age 8, works on an exercise machine on Nov. 29, 2006 with his brother Prakeet, who comes to the gym with Prashant for support, at the Youth Visions Fitness Center in Upper Marlboro, Maryland. (TIM SLOAN/TIM SLOAN/AFP/Getty Images)

Weighing in at 68kg , Prashant Kumar, left, age 8, works on an exercise machine on Nov. 29, 2006 with his brother Prakeet, who comes to the gym with Prashant for support, at the Youth Visions Fitness Center in Upper Marlboro, Maryland.(TIM SLOAN/TIM SLOAN/AFP/Getty Images)

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What's more, obesity appears to spread like a virus, in what researchers call the social-multiplier effect: People put on more weight when their family and friends are also heavy.

Still, even experts can't agree on what works, so governments have relied on blunt instruments such as shock-value billboards or promoting nutrition and fitness guidelines.

A different philosophy comes from the field of behavioural economics, which suggests that people can be "nudged" into better choices with incentives and disincentives, environmental design, peer pressure or even simply posing the right question at the right time - for instance, asking people to be organ donors when they renew their driver's licences.

The idea, also described as "choice architecture," was popularized in the influential 2008 book Nudge by economist Richard H. Thaler and legal scholar Cass R. Sunstein. One of its biggest fans is British Prime Minister David Cameron, who commissioned a small group of civil servants and academics as a Behavioural Insights Team to advise his cabinet.

Its report in December suggested, among other initiatives, installing swipe machines that would allow students to collect movie tickets for walking to school, and rebranding fruits and vegetables as "sports candy."

Crunch the numbers: What works?

Still, it's not clear that just building sidewalks will lead to weight loss, Cornell University economist John Cawley points out. Hiding the junk food and showcasing the baby carrots in school cafeterias is good practice, but will that stop the kids from dashing across the street to McDonald's?

"This is a complex world we live in now," says Tim Lang, a professor of food policy at City University London. "We've got to do a massive attempt to change how people eat and what they eat. This is long-term stuff."

Last fall, the Organization for Economic Co-operation and Development released a 270-page report that priced out current policy approaches against results. While not dismissing the usefulness of public-education campaigns - by far the most common approach in the countries studied - it found that overall those costs outweighed their benefits.

In most cases, mass-media campaigns, much like warning labels on cigarette boxes, drive home a message most consumers already know (while potentially, as Mr. Seeman points out, fostering a stigma a that prevents people from seeking help).

Food labelling, a cost borne primarily by private industry, also showed only marginal behavioural changes - though Prof. Cawley says it's cheap enough to be worthwhile even to influence just a small segment of the population.

School-based interventions were harder to cost out, since the benefits are long-term. But the report's authors did find it made sense to educate a captive audience, young enough to still be setting lifestyle patterns.

Restricting food ads during children's programming, as Quebec has done for decades, leads to children seeing about 40 per cent fewer ads for fat, salt and sugar-rich foods. A good idea in principle, the report said, but Quebec's child-obesity rates have continued to rise with the rest of the country's.

The 'ka-ching!' route to a healthier population

But by far the most cost-effective measures, according to the report, were taxes and subsidies - while politically tricky, these approaches pay for themselves in reducing health costs.

A recurring debate, in many countries including Canada, centre on whether to apply a form of sin tax to soft drinks, for instance, making empty-calorie sweetened beverages more expensive, while bringing down the rising cost of fruits and vegetables - perhaps in the form of vouchers for low-income families.

In Britain, where the government has vowed to stop badgering people into weight loss, consumers may now receive coupons for healthy food, such as yogurt, as part of a $390-million (U.S.), industry-financed plan to promote health eating. Prof. Lang harshly criticizes the program as a "nanny state" approach that sidesteps the culpability of industry in stocking kitchens with cheap, high-fat food.

Taking a more sweeping strategy, Denmark is rolling out a "fat tax" on everything from chocolate bars to soda pop, fatty meat and whipped cream.

A Canadian study commissioned by the Heart and Stroke Foundation last fall proposed a 20-per-cent tax on sweetened "caloric" drinks. (The tax on cigarettes is about 70 per cent.) It argued that, unlike fast food, the drinks have no nutritional value. Therefore, it wouldn't unfairly penalize poorer families, who would have other choices.

Canadians might not see it on their scales in the morning, but one can of pop less per week per person could make "a significant population level change," says Guy Faulkner, a health-psychology professor at the University of Toronto who led the panel behind the report. (The OECD report estimates that a 10-per-cent price change typically produces a 2-per-cent shift in consumption in the opposite direction.)